The present invention relates to a compound girder forming a rigid connection for prefabricated ceiling panels. The present invention also relates to a method of manufacturing such girders. Such girders interconnect the ceiling panels in a bending stiff manner upon assembly at the building site.
Heretofore it was customary to assemble compound structures at the building site rather than to use predominantly preassembled components made in a factory. The methods assemblying the steel construction at the building site involve embedding the upper portions or rather the upper chord of a compound girder in the concrete as it is being poured at the building site. Several different types of steel structures are known for use in this kind of building construction.
German Pat. No. 76,977 discloses web plates which extend at a right angle out of a concrete panel, the free ends of the web plates extending out of the concrete panel are equipped with angle or sectional steel forming a chord, whereby the web plates may be subdivided by gaps providing a protection against thermal stresses.
U.S. Pat. No. 1,047,030 discloses a structure in which two L-sectional steel beams form a lower chord. Diagonal rods are secured between the L-sectional beams by means of screws. The upper ends of these diagonal rods are embedded in the concrete. The space between the lower chord and the concrete is filled by masonry.
It is also known that the steel structure of truss frameworks may comprise L-sectional steel lower chords, see for example U.S. Pat. Nos. 1,979,643 and 4,056,908, as well as German Patent Publication No. 2,123,351. U.S. Pat. No. 4,115,971 discloses structures with so-called honeycomb girders.
Contrary to the above prior art it is presently predominantly customary to prefabricate compound structural components. In this type of structure the individual ceiling panels are prefabricated in the factory or in the shop in sizes suitable for transportation. These panels are then assembled at the construction site to form, for example, a ceiling or a bridge. The individual ceiling panels may be steel reinforced concrete panels or they may be of the compound structure type or they may comprise compound structures including steel constructions extending in the direction of one axis only, whereby the assembly of the compound structure takes place at the building site, see for example German Publication No. 2,153,495 and U.S. Pat. No. 2,000,110.
In order to assemble such individual ceiling panels into a continuous ceiling it is customary to support the ceiling panels by means of so-called sleeper girders. It is possible to construct such sleeper girders together with the ceiling panels as a compound girder. It is the purpose of such a compound girder to interconnect two or more ceiling panels with each other in such a manner that a load bearing effect is achieved in a direction across the longitudinal extension of the joints between adjacent ceiling panels, whereby the ceiling panels themselves form a chord of the compound girder.
Compound girders including premanufactured ceiling panels are also known, for example, in connection with bridge construction. In this type of structure prefabricated ceiling or cover panels are secured to an I-beam in a manner secure against relative sliding between the panel and the I-beam by means of screws. This type of connection is described in more detail as an HV-connection in the German Industrial Standards DIN 1050. The gaps between adjacent panels are filled at the building site with a concrete mix. According to yet another prior art structure the sliding preventing screw connection is replaced by headed bolt dowels or by tholes welded in such a manner to the upper chord of the I-beam that these tholes or bolt heads reach into recesses of the cover panels. The recesses are then filled with concrete mix at the construction site. When the concrete in the joints between adjacent panels or in the recesses has hardened, one obtains a compound girder which provides a bending stiff connection of the prefabricated ceiling or cover panels. German Patent Publication No. 1,534,703 relating to a ceiling structure employing a compound girder construction method is based on this type of compound girders.
It is further known from German Patent Publication No. 2,526,278 also relating to a ceiling structure employing a compound girder method of construction, to provide the edges extending in the girder direction of ceiling panels, with angle steel and to weld headed bolts to the angle steel sections. These sections are screwed to an uninterrupted I-beam in a manner preventing any sliding between the sections and the I-beam.
Compound girders as described above have certain disadvantages. It is a common feature of all these girders that the connection, which is supposed to take up shearing loads between the chords is accomplished by connecting means such as a screw connection or a thole pin connection for preventing a relative sliding movement. Such a connection provides but one shearing plane in which the shearing loads are effective. Such single plane shear connections involve a high expenditure because a substantial number of connecting elements are required and must be installed.
It is further a basic disadvantage that the upper chord is weakened due to individual recesses which are filled with concrete at the construction site, such concrete having a lower strength characteristic. Similarly, it is disadvantageous if the upper chord cross-sectional area is diminished due to the space requirement for the screw connection components whereby the upper chord is weakened. This fact is especially disadvantageous for girders arranged along edges because the distribution of the forces effective on the upper chord into the ceiling panel exposes the compound girder to twisting due to torsion loads. Another disadvantage is seen in that in all of the mentioned structures the upper flange or chord of the I-beam is arranged below or under the ceiling panel which is disadvantageous in a static sense and its static load capacity is hence hardly utilized. Yet another specific disadvantage of the just mentioned compound girder is seen in the fact that an interruption of the bending stiffness of the ceiling panel in a direction extending perpendicularly to the compound girder axis cannot be avoided. Thus, it is, for example, not at all possible to use this type of ceiling panel in a cantilevered construction unless additional structural features are employed.